This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-200133, filed Jun. 30, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spin-valve transistor, which can be suitably employed, for example, in a magnetic head for reading high-density magnetic recording and in a high-density memory device such as a magnetic RAM (MRAM) and a magnetic ROM (MROM).
2. Description of the Related Art
Highly increased density and velocity of magnetic recording in recent years can be mainly attributed to progress in magnetic recording apparatuses, in particular, to progress in magnetic heads used for writing and reading of magnetic recording as well as to improvement in magnetic recording media. As the magnetic recording medium is increasingly reduced in size and improved in capacity, a relative velocity between the magnetic recording medium and the magnetic reading head is decreased correspondingly. In order to provide a high output even under such a situation, a giant magnetoresistive head (GMR head) comprising a spin-valve film has been developed as a new-type readout magnetic head. The GMR head has excellent characteristics in that it provides a high magnetoresistance ratio (MR ratio) compared with a conventional MR head. Recently, a GMR head of a tunnel junction type that is expected to exhibit even better characteristics is attracting a lot of attention.
Conventional magnetic recording media such as a magnetic disc is designed to function as a file memory, in which information stored therein is once read into a semiconductor memory (DRAM or SRAM) of a computer before the information is utilized. Certainly, the semiconductor memory has various excellent characteristics, but is accompanied with a defect in that it consumes high electric power for memory holding. In recent years, a flash memory and an FRAM, which require no electric power for memory holding, have been developed, but they have a drawback in that the number of rewrite operations is rather limited. On the other hand, endeavor to develop a magnetic memory (MRAM), which permits substantially infinite rewrite operations, has been started. In order to realize the MRAM, however, development of a material or a device capable of exhibiting a high MR ratio is desired.
Under the circumstances, a magnetic tunnel junction element is now attracting a lot of attention as an element exhibits a higher MR ratio than the conventional spin-valve film. The magnetic tunnel junction element or a combination of the magnetic tunnel junction element and a MOS transistor has been used in attempts to fabricate a magnetic head or a magnetic memory. Further, development of a spin-valve transistor capable of exhibiting a higher MR ratio than the magnetic tunnel junction element has also been started.
FIGS. 1A and 1B show examples of band diagrams of conventional spin-valve transistors.
The spin-valve transistor shown in FIG. 1A is of a type in which electrons are injected from an emitter via a tunnel junction into a base. This spin-valve transistor has a stacked structure of an Al emitter 11, a tunnel insulator 12, a base 13 comprising an Fe/Au/Fe spin-valve film, and an n-Si collector 14.
On the other hand, the spin-valve transistor shown in FIG. 1B is of a type in which electrons are injected from an emitter via a Schottky junction into a base. This spin-valve transistor has a stacked structure of an n-Si collector 21, a base 22 comprising a Co/Cu/Co spin-valve film, and an n-Si collector 23.
These spin-valve transistors are known to exhibit an extremely high MR ratio of several hundreds percent. However, these conventional spin-valve transistors have a defect in that a collector current (Ic) is extremely low, for example, in the level of about 10xe2x88x924 of an emitter current (Ie). This low ratio of collector current/emitter current (Ic/Ie) is undesirable in view of power consumption, operating speed, noise, and so on.
The reason why the collector current is extremely low in the conventional spin-valve transistors can be explained as follows. For example, in the case of the spin-valve transistor shown in FIG. 1A in which electrons are injected from the emitter via the tunnel junction into the base, angle dependency of the tunnel current can be represented by the following equation:
Jxcex8xe2x88x9d exp[xe2x88x92xcex22 sin2xcex8]xe2x80x83xe2x80x83(1)
where xcex24=2 ms2EF2/h2(Evxe2x88x92E); xcex8 is an angle formed between the normal line to the junction surface and a wavenumber vector of electrons; Jxcex8 is a current density in the direction of xcex8; m is the mass of electron; s is a width of the tunnel barrier; EF is Fermi energy; h is the Planck constant; Ev is a height of the tunnel barrier; and E is energy of tunnel electrons.
It will be seen from this equation that, when the direction of travel of electrons passing through the tunnel insulator is almost perpendicular to the junction surface, the tunnel current can be increased. Even in the case of the spin-valve transistor shown in FIG. 1B in which electrons are injected from the emitter via the Schottky junction into the base, the current can be increased when the direction of travel of electrons is almost perpendicular to the junction surface.
The spin-valve transistor is designed to operate based on spin-dependent scattering of electrons, which means that the manner of electron scattering changes depending on whether the spin directions are parallel or antiparallel in the two magnetic films of the spin-valve film included in the base. However, in the conventional spin-valve transistor, diffusive scattering is mainly caused within the magnetic layer (F) or at the interface between the magnetic layer (F) and the nonmagnetic layer (N) as shown in FIG. 2A. In this case, since the scattered electrons are incapable of flowing into the collector due to a strong diffraction effect at the interface between the base and the collector, the collector current is decreased. Therefore, in order to increase the collector current, it is necessary to reduce the diffusive scattering. However, there arises a problem that the MR ratio is also reduced if the diffusive scattering is reduced.
An object of the present invention is to provide a spin-valve transistor capable of exhibiting a high MR ratio and a high ratio of collector current/emitter current.
According to an aspect of the present invention, there is provided a spin-valve transistor comprising: an emitter, a base comprising a spin-valve film in which two magnetic layers are stacked with interposing a nonmagnetic layer between the two magnetic layers, and a collector, the spin-valve film having a stacked structure of M/A/Mxe2x80x2 or M/B/Mxe2x80x2 and the spin-valve film being (100)-oriented, where M and Mxe2x80x2 may be the same or different and individually comprises at least one element selected from the group consisting of Fe, Co, Ni and an alloy including Fe, Co, Ni; A comprises at least one element selected from the group consisting of Au, Ag, Pt, Cu and Al; and B comprises at least one element selected from the group consisting of Cr and Mn.
According to another aspect of the present invention, there is provided a spin-valve transistor, comprising: a spin-valve film comprising a first magnetic layer and a second magnetic layer stacked with interposing a nonmagnetic layer between the first and the second magnetic layers, the spin-valve film being (100)-oriented; a first electrode electrically connected to the first magnetic layer; a second electrode electrically connected to the second magnetic layer; wherein each of the first and the second magnetic layers comprises at least one element selected from the group consisting of Fe, Co, Ni and an alloy including Fe, Co, Ni; the nonmagnetic layer comprises at least one element selected from the group consisting of Au, Ag, Pt, Cu, Al, Cr and Mn.
According to still another aspect of the present invention, there is provided a spin-valve transistor manufactured by a process comprising: forming a first electrode on a substrate being (100)-oriented by epitaxial growth; forming a first magnetic layer on the first electrode; forming a nonmagnetic layer on the first magnetic layer; forming a second magnetic layer on the nonmagnetic layer; forming a second electrode on the second magnetic layer, wherein the first magnetic layer, the nonmagnetic layer and the second magnetic layer are formed with a deposition rate within the range of 0.01 nanometers per second to 0.1 nanometers per seconds.